The present invention relates to path switching at the time of a fault in a composite synchronous/asynchronous system comprised of a combination of a synchronous communication system and an asynchronous communication system.
Until now, communication networks have been constructed based on an asynchronous communication system (stuff multiplexing), but in recent years a synchronous communication system (synchronous multiplexing), different from the above-mentioned asynchronous communication systems, has been proposed for the purpose of globally standardizing the hierarchies in different countries. This new system is called the SDH (synchronous digital hierarchy) in Europe and Japan and is called SONET (synchronous optical network) in North America.
Therefore, future communication networks will probably be constructed by a composite synchronous/asynchronous system incorporating a synchronous communication system in an asynchronous communication system. The present invention refers to a responsive measure at the time of a fault in such a composite asynchronous/synchronous system.
FIG. 13 is a view showing an example of the general configuration of a synchronous communication system. In the figure, 10 shows the synchronous communication system as a whole. As illustrated, the synchronous communication system 10 adopts a so-called "ring structure" comprised of ring-like transmission lines 11 and 12 forming a closed loop. These ring-like transmission lines 11 and 12 have a plurality of offices 13A to 13D inserted in them (however, only four offices are shown here for simplification). These offices are called network nodes. A "through", "drop", or "insert" operation is performed for the path signal at each office (13). A "through" operation means the path signal running on the ring-like transmission line 11 passes through the office, a "drop" operation means that the signal destined for a home office is pulled in out of the path signal running on the ring-like transmission lies 11 and 12, and an "insert" operation means that a path signal from a home office is sent out to the ring-like transmission line 11. Note that there are a plurality of paths forming the transmission path of the path signal formed in the transmission line, the above-mentioned drop and insert operations are performed in units of paths, and the above-mentioned closed loop is formed for each path.
Referring to the figure, as one of the intrinsic features of a synchronous communication system, mention may be made of the route diversity. That is, in the figure, if a fault (shown by the x mark in the figure) occurs in the ring-like transmission line 12 at the bottom or at the office 13D inserted in part of the same, thanks to the above-mentioned route diversity function, the path signal running on the top ring-like transmission line 11 in the figure is selected from among the duplexed path signals at the office 13B acting as the path termination office in the case and therefore the path signal is rescued from the fault. This selection is performed by the path switch 17 in the office 13B.
If the above-mentioned synchronous communication system 10 is incorporated into a conventional asynchronous communication system 19, the former system 10 receives a signal of the asynchronous mode (asynchronous hierarchy signal) from the latter system 19 and the signal is transmitted from a connection office between the two systems, e.g., the office 13A, to a path termination office, e.g., the office 13B. The asynchronous mode signal is shown by Sa in the figure. It is converted to a synchronous mode path signal (synchronous hierarchy signal) Ss by the asynchronous/synchronous converter 14 in the office 13A, then is duplexed into two routes by a path branch unit 15 and sent out to the ring-like transmission lines 11 and 12.
The path termination office 13B receives at the path switch 17 a signal Ss from the ring-like transmission line 11 through the path monitors 16 corresponding to the two routes. The path switch 17 selects one of the routes. If a path monitor 16 detects some sort of fault in the selected path signal, the path switch 17 changes over to the other route. Note that the path monitor 16 is activated when an AIS (alarm indication signal) showing the suspension of the path signal or a fault in the path signal is detected and switches the path switch 17.
The synchronous mode path signal Ss passing through the path switch 17 is returned to the asynchronous mode signal Sa by the synchronous/asynchronous converter 18 and sent to the communication network constructed under the asynchronous communication system.
FIG. 14 is a view showing a conventional example in which synchronous and asynchronous communication systems are merged. In the synchronous communication system 10 in the composite asynchronous/synchronous system, provision is made of a route diversity function as mentioned above (ring-like transmission lines 11 and 12). Further, the asynchronous communication system 19 is also provided with a similar route diversity function by the redundancy lines (pair of optical fibers) 20. Note that 21 is a redundancy line forming unit and that 22 is a redundancy line termination unit. Part of the redundancy lien termination unit forms part of the above-mentioned connection office 13A.
As clear from FIG. 14, both of the synchronous communication system 10 and the asynchronous communication system 19 can individually enjoy the merits of the route diversity function. However, there is the disadvantage that no protection route can be secured against a fault (shown by the x mark in the figure) occurring in the connection office 13A at a connecting point between these systems 10 and 19. Accordingly, a composite asynchronous/synchronous system which features an improvement on this point has been required.
FIG. 15 is a view of a composite asynchronous/synchronous system improved over the system shown in FIG. 14. According to the configuration in FIG. 15, the connection office 13A shown in FIG. 14 is duplexed so that the connection office 13A and another connection office 13A' are mounted, the connection office 13A is connected to the redundancy line 20a and the connection office 13A' is connected to the redundancy line 20b, and therefore route diversity is formed at the merged portion of the synchronous communication system 10 and the asynchronous communication system 19.
However, there is a problem with the system shown in FIG. 15. By adopting the configuration shown in FIG. 15, route diversity can be formed spanning the synchronous communication system 10 and the asynchronous communication system 19. Under such a configured composite system, if a fault were to occur on the ring-like transmission line 11 (or 12) in the synchronous communication system 10, then as mentioned earlier the fault would be detected by the path monitor 16 and the path monitor 16 would switch the path switch 17 to the ring-like transmission line 12 (or 11) to rescue the system 10 from the fault.
Even if a fault occurred on the redundancy line 20a (or 20b) in the asynchronous communication system 19, however, the path switch 17 would not switch to the ring-like transmission line 12 (or 11). Accordingly, when a fault occurs in the asynchronous communication system 19, the route diversity function of the composite asynchronous/synchronous system will not work. That is, a protection route cannot be ensured and this is the problem in the system shown in FIG. 15.
This type of problem occurs because the fault occurring at the asynchronous communication system 19 side is not detectable at the synchronous communication system 10 side.